He can produce blues ONLY if bred to a blue or a splash. If he has a black hen, you'd only get black chicks, never blue.
Blue, black and splash are compatible breeding colors, has nothing to do with being "purebred". Blue x black can give you black or blue chicks. Blue x blue can give you blue, black OR splash chicks. It's just a product of the blue gene and how it works. See the rooster in my avatar, a Blue Orpington? I've had blue Orps, blue Rocks and blue Ameraucanas and know a little something about breeding blue, black and splash. If they think you can get a blue chick out of two black birds, they are mistaken. Some blues can be so dark, you think they are black and make a mistake, but they won't have that sheen that he does.